The present invention generally relates to the field of superconducting microwave circuits operating in the microwave regime, as well as related devices and methods of fabrication thereof. Embodiments of the present invention improve quality factors of resonators and/or qubit lifetimes (for quantum information processing devices) through the reduction of microwave photon interactions with the surrounding materials.
The reduction of microwave photon interactions with the environment is needed for increasing the fidelity of quantum gate operations and reaching thresholds to run quantum algorithms. The main source of unwanted microwave photon interactions (e.g., microwave photon losses, the photon phase effects, etc.,) in superconducting circuits is believed to arise due to electromagnetic coupling to imperfections or adsorbates at material interfaces in the vicinity of the qubit. Three types of interfaces are typically identified: the (i) substrate-to-air (SA), substrate-to-metal (SM), and metal-to-air (MA) interfaces. The probability for a microwave photon to be absorbed at any of these interfaces is usually assumed to be proportional to the integrated energy density (ED) of the electric field distribution of the corresponding microwave excitation over the volume of the interface, referred to as the participation ratio of the interface. The microscopic origin is often attributed to the presence of an ensemble of two-level fluctuators in amorphous materials such as native oxides or surface adsorbate layers that couple to the microwave field through their dipole moments. Similarly, magnetic two-level fluctuators such as adsorbed oxygen molecules or some hydrogen bond species are believed to limit the coherence (T2) times of flux tunable qubits. To mitigate this issue, planar qubit and coplanar waveguide (CPW) designs have been made larger (lateral sizes of several hundreds of micrometers) such that the participation of interface layers in the electromagnetic field energy density (EMFED) would become negligible and interface loss is reduced. This has led to improvements of qubit T1 times (to more than 100 μs).
Qubit lifetimes need to be further improved to achieve efficient quantum computing (QC) architectures. However, qubit size cannot be indefinitely increased, due to the need of scalability and packaging of multiqubit chips. Moreover, increasing the qubit size will only yield a logarithmic improvement of the interface participation.
The MA interface is known to have very low participation in the EMFED. The SM interface can be engineered and/or improved during fabrication, because it is not exposed to ambient conditions during packaging and transfer of the qubit chips into the measurement system (dilution refrigerator). Finally, the dominating SA interface cannot easily be improved. One reason is that the known “passivation” methods happen to deteriorate the qubit lifetimes.